Doubling up. When I first started work in the 60s a mate at work suggested exactly this to me. Foolproof, he said. We went to a casino with something like £20 between us (this was as far as I remember 1967 or 68). We came out that night with £110 each, which seemed a fortune at the time. So of course went back the next night… and lost it all. Yes, you have to have a huge amount of money to make it work, the casino has to have no limit to how many times you can double up and thirdly there’s always the chance that the ball lands on zero in which case the bank wins.